The majority of cosmetic and pharmaceutical emulsions are of the oil-in-water type, i.e., the oil phase (“disperse phase”) is very finely distributed in the form of small droplets in the water phase (“coherent phase”). The viscosity of emulsions which consist only of water, oil and emulsifier, and whose content of disperse phase is below 60%, by weight, is equal to the viscosity of the coherent phase, and, in the case of oil-in-water emulsions, is thus equal to that of water. For reasons of feel on the skin, cosmetic emulsions on average comprise not more than 30% of oil phase, i.e., cosmetic emulsions are typically water-thin. Since, however, the consumer generally desires a lotion-like (high-viscosity) to cream-like (semisolid) consistency, and also the stability of emulsions increases with the viscosity of the coherent phase, the “thickening” of oil-in-water emulsions is essential. For this purpose there are two fundamentally different methods which can be combined with one another. The first method is based on the fact that certain oil-in-water emulsifiers are able, together with so-called “hydrophilic waxes”, to form liquid-crystalline (lamellar) structures in the coherent water phase. Moreover, this first method forms a three dimensional network which leads to a large increase in the viscosity of the emulsion; keeps the oil droplets separate from one another; and thus improves the stability of the emulsion. Examples of “hydrophilic waxes” are stearyl alcohol, stearic acid and glyceryl stearate.
The other method is based on the ability of so-called “hydrocolloids” to take up and bind many times their own weight of water and thus lead to thickening of water. Examples of such water-swellable organopolymers are crosslinked polyacrylates (“carbomers”) and polysaccharides, for example, xanthan gum. A disadvantage of these two thickening methods mentioned above is that the substances used therein can adversely affect the feel on the skin during or after application of the emulsions. Thus, for example, in the presence of relatively large amounts of hydrophilic waxes, the emulsions can only be spread with difficulty, and a dull, waxy feel on the skin often remains. On the other hand, the water-swellable organopolymers also display disadvantages in application properties. Thus, for example, in the case of carbomers, the so-called “quick-breaking effect” is observed. The “quick-breaking effect” is understood as the phenomenon where, in the case of contact of the emulsion with the electrolytes of the skin, the emulsion immediately breaks. This phenomenon is evident from an “aqueous sliding away” upon rubbing in and is often perceived as unpleasant.
For the preparation of oil-in-water emulsions, use is usually made of emulsifiers whose HLB value is between 8 and 18. The HLB value is a dimensionless parameter for characterizing surfactants and describes the ratio of the hydrophilic portion to the lipophilic portion in the molecule (HLB=hydrophilic-lipophilic balance). Thus, on the basis of numerous experiments by Griffin (J. Soc. Cosmet. Chem. 1949, 1, 311), it has been found that surfactants with an HLB value of 3 to 6 are suitable as water-in-oil emulsifiers, those with an HLB value of 6 to 8 are suitable as wetting agents, and surfactants with an HLB value of greater than 8 are suitable as oil-in-water emulsifiers. In the simplest case, the HLB value is calculated from the percentage proportion of the hydrophilic part of an emulsifier, for example, the polyethylene glycol part, by dividing this by 5. Thus, for example, the hydrophilic portion in the addition product of 20 mol of ethylene oxide (MW=880 g/mol) to stearic acid (284 g/mol) is 76%, corresponding to an HLB value of 15 (=76/5). This HLB concept has originally been limited to nonionogenic substances which contain no atoms other than carbon, hydrogen and oxygen. In addition, this FLB value definition does not apply exactly for substances whose hydrophilic part also contains propylene glycol units in addition to ethylene glycol units.
A disadvantage of emulsifiers with an HLB value of significantly greater than 8 is that such emulsifiers are less mild than emulsifiers with a lower HLB value. In addition, because of their higher hydrophilicity, emulsifiers having a HLB value of greater than 8 are more readily redispersible, i.e., they can be more readily washed off from the skin again with water, which, for example, in the case of sunscreen formulations that are supposed to be water-resistant, is undesired. Conversely, emulsifiers with an HLB value of around 8 and below form a hydrophobic film on the skin which protects the skin from excessive water loss and thus has a care effect. This is probably the main reason for the fact that water-in-oil emulsions, which require emulsifiers with an HLB value of less than 8, have a stronger care effect than oil-in-water emulsions that contain hydrophilic emulsifiers. However, oil-in-water emulsions are usually preferred by the consumer since oil-in-water emulsions can be spread more readily because of the aqueous external phase.
Oil-in-water emulsions which comprise polyether siloxanes are known from the prior art, as is shown below.
EP 0 154 837 A2 describes low-viscosity oil-in-water emulsions with a combination of a comb-like, terminally capped polyether siloxane, a surfactant with an HLB value of not less than 10, and a fatty alcohol as emulsifiers which have a low oil phase content and whose oil phase consists predominantly of silicone oil and, in addition, the water phase contains ethanol.
EP 0 279 319 A describes pigment-containing oil-in-water emulsions with a polyether siloxane as emulsifier, the polyether radical of which contains a maximum of 50 mol % of polyoxypropylene units, and whose oil phase consists predominantly of unmodified or alkyl-modified silicone oils.
EP 0 516 547 A describes oil-in-water emulsions with a comb-like polyether siloxane with an HLB value of from 9 to 12 as emulsifier, the polyether of which consists exclusively of polyethylene oxide with a terminal OH group. The oil phase consists of a chain-shaped or a cyclic siloxane.
DE 4 41 799 C1 describes cosmetic compositions that are in the form of two separate phases which are optically separate from one another, but can be combined by shaking directly prior to application to give a homogeneous emulsion and, following application, rapidly separate again into separate phases. The emulsifier used is a comb-like polyether siloxane.
EP 0 627 259 A2 discloses that silicone polyethers with an HLB value between 4 and 7 can also be used to prepare silicone-in-water emulsions. These emulsions are prepared by stirring an oil phase which consists of silicone oil and a first silicone polyether into a water phase which contains a second silicone polyether. Both silicone polyethers are comb-like in structure.
The prior art can be summarized as follows: oil-in-water emulsions with silicone polyethers as emulsifiers are known, in which the oil phase consists for the most part of silicone oils and the silicone polyether is of comb-like structure.